This invention relates to the fabrication of three-dimensional objects using additive process modeling techniques. More particularly, the invention relates to forming three-dimensional objects by depositing solidifiable material in a predetermined pattern and providing support structures to support portions of such a three-dimensional object as it is being built.
Additive process modeling machines make three-dimensional models by building up a modeling medium, based upon design data provided from a computer aided design (CAD) system. Three-dimensional models are used for functions including aesthetic judgments, proofing the mathematical CAD model, forming hard tooling, studying interference and space allocation, and testing functionality. One technique is to deposit solidifiable modeling material in a predetermined pattern, according to design data provided from a CAD system, with the build-up of multiple layers forming the model.
Examples of apparatus and methods for making three-dimensional models by depositing layers of solidifiable modeling material from an extrusion head are described in Crump U.S. Pat. No. 5,121,329, Batchelder, et al. U.S. Pat. No. 6,303,141, Crump U.S. Pat. No. 5,340,433, Batchelder, et al. U.S. Pat. No. 5,402,351, Danforth, et al. U.S. Pat. No. 5,738,817, Batchelder, et al. U.S. Pat. No. 5,764,521 and Swanson et al. U.S. Pat. No. 6,004,124, all of which are assigned to Stratasys, Inc., the assignee of the present invention. The modeling material may be supplied to the extrusion head in solid form, for example in the form of a flexible filament wound on a supply reel or in the form of a solid rod, as disclosed in U.S. Pat. No. 5,121,329. As described in U.S. Pat. No. 4,749,347, modeling material may alternatively be pumped in liquid form from a reservoir. In any case, the extrusion head extrudes molten modeling material from a nozzle onto a base. The extruded material is deposited layer-by-layer in areas defined from the CAD model. A solidifiable material which adheres to the previous layer with an adequate bond upon solidification is used as the modeling material. Thermoplastic materials have been found particularly suitable for these deposition modeling techniques.
Examples of apparatus and methods for making three-dimensional models by depositing solidifiable material from a jetting head are described, for example, in Helinski U.S. Pat. No. 5,136,515, Masters U.S. Pat. No. 4,665,492 and Masters U.S. Pat. No. 5,216,616. Particles are directed to specific locations in a predetermined pattern as defined by a CAD model, and deposited and built up to construct the desired object.
In creating three dimensional objects by additive process techniques, such as by depositing layers of solidifiable material, it is the rule rather than the exception that supporting layers or structures must be used underneath overhanging portions or in cavities of objects under construction, which are not directly supported by the modeling material itself. For example, if the object is a model of the interior of a subterranean cave and the cave prototype is constructed from the floor towards the ceiling, then a stalactite will require a temporary support until the ceiling is completed. Support layers or structure may be required for other reasons as well, such as allowing the model to be removed from a base, resisting a tendency for the model to deform while partially completed, and resisting forces applied to a partially completed model by the construction process.
A support structure may be built utilizing the same deposition techniques and apparatus by which the modeling material is deposited. The apparatus, under appropriate software control, produces additional geometry acting as a support structure for the overhanging or free-space segments of the object being formed. Support material is deposited either from a separate dispensing head within the modeling apparatus, or by the same dispensing head that deposits modeling material. The support material is chosen so that it adheres to the modeling material. Anchoring the model to such support structures solves the problem of building the model, but creates the additional problem of removing the support structure from the finished model without causing damage to the model.
The problem of removing the support structure has been addressed by forming a weak, breakable bond between the model and the support structure, such as is described in Crump, et al. U.S. Pat. No. 5,503,785. The ""785 patent discloses a process by which a material that forms a weak, breakable bond with the modeling material is selected as a release material. The release material is deposited along the interface between the object and its support structure in a layered fashion or as a coating, permitting the support structure to be broken away after formation of the object. The support structure may be formed of the modeling material or it may be formed of the release material.
The ""785 patent discloses various combinations of materials that may be used as modeling and release materials. For instance, the ""785 patent discloses that a soluble release material may be utilized, so that any such material remaining on the model after the support is broken away can be removed by placing the model in a bath. Water soluble wax, polyethylene oxide and glycol-based polymers, polyvinyl pyrrolidone-based polymers, methyl vinyl ether, maleic acid-based polymers, polyoxazoline-based polymers and polyquaternium II are disclosed, as well as solvent-soluble acrylates and stearic and azelaic acids. Soluble supports can eliminate scarring of the model surface and the need to use force in removing supports.
In extrusion based systems, a variation of applying release material in layers has been implemented, in which the release material is applied in short bead segments (termed xe2x80x9cperforationsxe2x80x9d) between the support structure and the model under construction. The perforations reduce adhesion of the support layer by limiting the area of contact with the model, to aid in the removal of breakaway supports.
There is a continuing need to provide a support structure that releases from a three-dimensional model without the application of force and that will not mar the model surface finish, and that further has good mechanical strength and is compatible with the modeling process and the modeling material.
The present invention is an improved deposition modeling process which uses an alkali-soluble thermoplastic material for forming an alkali-soluble support structure for a three-dimensional object under construction or for forming an alkali-soluble three-dimensional object. The alkali-soluble material comprises a base polymer containing a carboxylic acid, and a plasticizer. In the preferred embodiment, the carboxylic acid is methacrylic acid and the base polymer further contains an alkyl methacrylate, preferably methyl methacrylate. The alkyl methacrylate comonomer provides thermal and toughness properties suitable for depositing modeling, while the plasticizer reduces the viscosity and increases the melt flow index of the base polymer. A support structure or object formed from the alkali-soluble thermoplastic material dissolves when placed in an alkaline bath.